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From Forts to Fossils: Formations of the Crown Point Historic Site and Their Role in Ordovician Reef Environments

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Alivia Hartz

Authors:

Hartz, Alivia Aspen¹, Zeltsar, Max K.², Mychajliw, Alexis M.³, Wittmer, Jacalyn Marie⁴

(1) Department of Earth and Climate Sciences, Middlebury College, Middlebury, Vermont, USA; Department of Biology, Middlebury College, Middlebury, Vermont, USA, (2) Department of Evolution, Ecology, and Organismal Biology, UC Riverside, Riverside, California, USA, (3) Department of Biology, Middlebury College, Middlebury, VT, USA; Program in Environmental Studies, Middlebury College, Middlebury, Vermont, USA, (4) Department of Geological Sciences, SUNY Geneseo, Geneseo, NY, USA,

Abstract:

Crown Point Historic Site in Crown Point, New York is home to four, Middle to Upper Ordovician fossil-rich formations that serve as the building stone for the forts at the historic site (Crown Point, Valcour, Orwell, and Glens Falls Formations). Although the units at Crown Point have been previously described, we present an in-depth study using paleoecological techniques—such as biovolume analysis—to reassess paleoenvironmental interpretations at the Crown Point Historic Site and evaluate their relevance to the Great Ordovician Biodiversification Event. The scope of this study is to characterize the Valcour and Orwell Formations, focusing on the roles and relative abundances of both reef framework builders (tabulate corals and stromatoporoids) and reef dwelling (crinoids, gastropods, brachiopods) organisms in a paleoecological context. Field methodology involved constructing detailed stratigraphic sections to define facies transitions and boundaries. Additionally, 50 x 50-centimeter point-grids were used along transects at identified fossil horizons to conduct individual fossil counts and biovolume assessments. The Orwell Formation has previously been described as massive and lacking significant faunal evidence; however, this study identifies five distinct fossil-rich lithofacies: (1) homogenous lime mud wackestone with rare bioturbation and fossil fragments; (2) heavily bioturbated lime mud wackestone containing common, identifiable fossils of crinoids, gastropods, and brachiopods; (3) lime mud bindstone characterized by laterally continuous laminar stromatoporoids; (4) thinly-bedded packstone rich in brachiopod, crinoid, bryozoan, stromatoporoid, and gastropod fossils; and (5) massive, fine-grained mudstone containing few fragmented fossils. The predominance of overturned stromatoporoids and tabulate corals, along with the general absence of in situ framework organisms across multiple horizons in the Orwell Formation, suggests a biohermal origin rather than a true reef structure. In the Valcour Formation, there is evidence of channel-fill that is variable in thickness but laterally continuous. Maclurites are abundant across this layer, possibly indicating the presence of a death assemblage. Comparable sites, both in terms of stratigraphic and faunal composition, can be found across the Champlain Valley of Vermont and eastern New York. These data help place the Crown Point Historic Site within the broader paleogeographic context of northeastern Ordovician reef habitats, offering new insights into reefal ecological successional stages and their relationship to the Great Ordovician Biodiversification Event.